Solid polymer electrolyte fuel cell

ABSTRACT

Disclosed is a polymer electrolyte fuel cell having an improved separator plate. The fuel cell comprises a solid polymer electrolyte membrane; an anode and a cathode sandwiching the solid polymer electrolyte membrane therebetween; an anode-side conductive separator plate having a gas flow path for supplying a fuel gas to the anode; and a cathode-side conductive separator plate having a gas flow path for supplying an oxidant gas to the cathode, wherein each of the anode-side and cathode-side conductive separator plates is composed of a metal and a conductive coat which has resistance to oxidation and covers a surface of the metal. Alternatively, the above-mentioned separator plates are formed of a metal and a coat having resistance to oxidation and have roughened surfaces with recessions and protrusions, and portions of a top surface of the protruding portions, which lack the coat, are electrically connected to an electrode.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to solid polymer electrolyte fuel cellsused for portable power sources, electric vehicle power sources,domestic cogeneration systems, etc.

2. Background Art

A fuel cell using a solid polymer electrolyte generates electric powerand heat simultaneously by electrochemically reacting a fuel gascontaining hydrogen and an oxidant gas containing oxygen such as theair. This fuel cell is basically composed of a polymer electrolytemembrane for selectively transporting hydrogen ions, and a pair ofelectrodes, namely, an anode and a cathode, formed on both surfaces ofthe polymer electrolyte membrane. The above-mentioned electrode usuallyincludes a catalyst layer which is composed mainly of carbon particlescarrying a platinum metal catalyst and formed on the surface of thepolymer electrolyte membrane, and a diffusion layer which has both gaspermeability and electronic conductivity and is formed on the outersurface of this catalyst layer.

Moreover, a gas sealing material or gaskets are arranged on theperipheral portions of the electrodes with the polymer electrolytemembrane therebetween so as to prevent a fuel gas and an oxidant gassupplied to the electrodes from leaking out or prevent two kinds ofgases from mixing together. These sealing material and gaskets areassembled into a single part together with the electrodes and polymerelectrolyte membrane in advance. This part is called the “MEA”(electrolyte membrane and electrode assembly). Disposed outside of theMEA are conductive separator plates for mechanically securing the MEAand for connecting adjacent MEAs electrically in series, or in parallelin some case. A portion of the separator plate, which is in contact withthe MEA, is provided with a gas flow path for supplying a reacting gasto the electrode surface and for removing a generated gas and an excessgas. Although the gas flow path can be provided separately from theseparator plate, grooves are usually formed on the surface of eachseparator to serve as the gas flow path.

In order to supply the fuel gas and oxidant gas to such grooves, it isnecessary to branch pipes for supplying the fuel gas and oxidant gas,respectively, according to the number of separator plates to be used,and to use piping jigs for connecting an end of the branch directly tothe grooves of the separator plate. This jig is called “manifold”, and atype of manifold which directly connects the supply pipes of the fuelgas and oxidant gas to the grooves as mentioned above is called the“external manifold”. There is a type of manifold, called the “internalmanifold”, with a more simple structure. The internal manifold is one inwhich through apertures are formed in the separator plates having a gasflow path and the inlet and outlet of the gas flow path are extended tothe apertures so as to supply the fuel gas and oxidant gas directly fromthe apertures.

Since the fuel cell generates heat during operation, it is necessary tocool the cell with cooling water or the like so as to keep the cell ingood temperature conditions. In general, a cooling section for feedingthe cooling water is provided for every one to three cells. There are atype in which the cooling section is inserted between the separatorplates and a type in which a cooling water flow path is provided on therear surface of the separator plate so as to serve as the coolingsection, and the latter is often used. The structure of a common cellstack is such that the MEAs, separator plates and cooling sections areplaced one upon another to form a stack of 10 to 200 cells, and thiscell stack is sandwiched by end plates, with a current collector plateand an insulating plate between the cell stack and each end plate, andsecured with a clamping bolt from both sides.

In such a solid polymer electrolyte fuel cell, the separator plates needto have a high conductivity, high gas tightness with respect to a fuelgas and oxidant gas, and high corrosion resistance against a reaction ofhydrogen/oxygen oxidation-reduction. For such reasons, conventionalseparator plates are usually formed from carbon materials such as glassycarbon and expanded graphite, and the gas flow path is formed by cuttingthe surface of the separator plate, or by molding with a mold when thematerial is expanded graphite.

In a conventional method including cutting a carbon plate, it isdifficult to reduce the cost of the material of the carbon plate and thecost of cutting the carbon plate. Besides, a method using expandedgraphite requires a high cost of material, and it has been consideredthat the high cost of material prevents a practical use of this method.

In resent years, there have been attempts to use a metal plate such asstainless steel in place of the conventionally used carbon material.

However, in the above-mentioned method using a metal plate, since themetal plate is exposed to an acidic atmosphere of the pH of 2 to 3 athigh temperatures, the corrosion or dissolution of the metal plate willoccur when used in a long time. The corrosion of the metal plateincreases the electrical resistance in the corroded portion anddecreases the output of the cell. Moreover, when the metal plate isdissolved, the dissolved metal ions diffuse into the polymer electrolytemembrane and are trapped by the ion exchange cite of the polymerelectrolyte membrane, resulting in a lowering of the ionic conductivityof the polymer electrolyte membrane. For these causes, when a cell inwhich a metal plate is used as it is for a separator plate is operatedfor a long time, a problem arises that the power generating efficiencyis gradually lowered.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to improve a separator plate foruse in fuel cells and provide a separator plate which maintains chemicalinactivity even when its surface to come in contact with a gas isexposed to an acidic atmosphere, suppresses corrosion and dissolution,and has good conductivity by using a metal that can readily be processedas a material.

The present invention provides a solid polymer electrolyte fuel cellcomprising: a solid polymer electrolyte membrane; an anode and a cathodesandwiching the solid polymer electrolyte membrane therebetween; ananode-side conductive separator plate having a gas flow path forsupplying a fuel gas to the anode; and a cathode-side conductiveseparator plate having a gas flow path for supplying an oxidant gas tothe cathode, wherein each of the anode-side and cathode-side conductiveseparator plates is composed of a metal and a conductive coat which hasresistance to oxidation and covers a surface of the metal.

The conductive coat is preferably selected from the group consisting ofa carbonaceous coat, a conductive inorganic compound coat, and ametal-plated coat containing particles of a water repellent material.

A preferred conductive separator plate is composed of a spongy metal anda carbon powder layer which is filled into the spongy metal and coversthe surface of the spongy metal.

Another preferred conductive separator plate is composed of a metalplate and a conductive coat covering the surface of the metal plate,wherein the conductive coat is a conductive inorganic compound selectedfrom the group consisting of oxides, nitrides and carbides.

Still another preferred conductive separator plate is composed of ametal plate and a conductive coat covering the surface of the metalplate, wherein the conductive coat is made of a metal-plated coatcontaining particles of a water repellent material.

The present invention provides a solid polymer electrolyte fuel cellcomprising anode-side and cathode-side conductive separator plates, eachof which is formed by a metal whose surface is covered with a coathaving resistance to oxidation, wherein at least surfaces of theseparator plates which face an anode and cathode are roughened to haverecessions and protrusions, and portions of the top surface of theprotruding portions, which lack the coat, are electrically connected tothe anode and cathode, respectively.

Moreover, the present invention provides a solid polymer electrolytefuel cell comprising anode-side and cathode-side conductive separatorplates, each of which is formed by a metal whose surface is covered witha coat having resistance to oxidation, wherein portions of the separatorplates' surface facing an anode and cathode, which lack the coat, areelectrically connected to the anode and cathode, respectively, throughconductive particles interposed between the separator plate and theanode and between the separator plate and the cathode.

The present invention provides a conductive separator plate composed ofa metal plate having grooves or ribs for guiding a fuel gas or oxidantgas on its surface facing an electrode, and an insulating sheet whichforms a gas flow path for guiding the fuel gas or oxidant gas from a gassupply side to a gas discharge side on a surface of the metal plate incooperation with the grooves or ribs and has elasticity to function as agasket for preventing the fuel gas or oxidant gas from leaking out ofthe gas flow path.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross sectional view depicting essential sections of a fuelcell according to an example of the present invention.

FIG. 2 is a plan view of an anode-side separator plate of the fuel cell.

FIG. 3 is a plan view of a cathode-side separator plate of the fuelcell.

FIG. 4 is a plan view of a metal plate constituting the anode-sideseparator plate.

FIG. 5 is a plan view of an insulating sheet constituting the anode-sideseparator plate.

FIG. 6 is a drawing showing the output characteristics of fuel cells ofExample 1 of the present invention and a comparative example.

FIG. 7 is a depiction showing an electrical contact section between aseparator plate and an electrode according to another example of thepresent invention.

FIG. 8 is a drawing showing the current-voltage characteristics of fuelcells of Example 9 of the present invention and a comparative example.

FIG. 9 is a drawing showing the current-voltage characteristics of fuelcells of Example 10 of the present invention and a comparative example.

FIG. 10 is a depiction showing an electrical contact section between aseparator plate and an electrode according to still another example ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

A separator plate of the present invention is basically composed of ametal plate whose surface is covered with a coat having resistance tooxidation. Besides, this metal plate is provided with ribs or groovesfor forming a gas flow path by, for example, press working.

A preferred separator plate of the present invention is composed of acombination of the worked metal plate having ribs or grooves for guidinga fuel gas or oxidant gas on its surface facing an electrode and aninsulating sheet which has elasticity and functions as a gasket. Theinsulating sheet forms a gas flow path for guiding the fuel gas oroxidant gas from a gas supply side to a gas discharge side incooperation with the ribs or grooves of the metal plate, and functionsas a gasket for preventing the fuel gas or oxidant gas from leaking outof the gas flow path.

In a preferred embodiment, the coat having resistance to oxidation whichcovers the surface of the metal plate is a coat having specifiedconductivity as described below.

In another embodiment, the coat having resistance to oxidation is a coathaving poor conductivity or insulating properties. In this case, as tobe described later, the separator plate is electrically connected to ananode or cathode through a portion lacking the coat.

As the above-mentioned metal, a metal plate such as stainless steel andaluminum, which has excellent conductivity and allows easy formation ofribs or grooves serving as a gas flow path by press working, etc., isused. For the conductive coat to be coated on such a metal plate, it ispreferable to use a coat of a conductive inorganic compound and ametal-plated coat containing particles of a water repellent material.

As the inorganic compound forming the conductive coat, it is preferableto use oxides, such as indium-doped tin oxide Sn(In)O₂, lead oxides PbOand PbO₂, nitrides, such as TiN and TiAlN, and carbides such as SiC. Inorder to form such a conductive coat on the metal plate surface,methods, such as vacuum evaporation, electron beam evaporation,sputtering and high frequency glow discharge decomposition, are used.The thickness of such a conductive coat is preferably in the range of500 Å to 5 μm from the viewpoint of the possession of both of corrosionresistance and conductivity.

The metal-plated coat containing particles of a water repellent materialis obtained by including particles of a water repellent material in acoat plated with a corrosion-resistant metal such as gold, silver,nickel and chrome. Materials used as the water repellent material arefluorocarbon resin-based water repellent materials, such as pitchfluoride, fluorinated graphite, polytetrafluoroethylene,tetrafluoroethylene-hexafluoropropylene copolymer andtetrafluoroethylene-perfluoroalkyl vinyl ether copolymer. Themetal-plated coat containing particles of a water repellent material isobtained by dispersing the particles of the water repellent material ina plating bath for obtaining the metal plating and by performingelectroplating in this plating bath. The particle diameter of theparticles of the water repellent material used here is in the range of0.05 to 50 μm, preferably 0.5 to 10 μm, and the thickness of the platedcoat is preferably in the range of 0.5 to 10 μm. An appropriate pH ofthe plating bath is in the range of 3 to 6, and an appropriate currentdensity is in the range of 0.1 to 1 A/cm².

The conductive separator plate of the present invention can also becomposed of a spongy metal and a carbon powder layer which is filledinto the spongy metal and covers the surface of the spongy metal.Specifically, the carbon powder is made into a paste with water or anaqueous solution of a viscous agent such as carboxymethyl cellulose, andthis paste is filled into the spongy metal. Then, the resulting spongymetal is pressed by application of pressure so as to form a separatorplate having a gas flow path. In this manner, a separator plate, whichis filled with the carbon powder and has a surface covered with thecarbon powder layer having excellent resistance to corrosion, isfabricated.

The following description will explain the cases where theoxidation-resistant coat is a coat having poor conductivity orinsulating properties, and the separator plate is electrically connectedto the anode or cathode through a portion lacking the coat.

First, in the first case, the metal plate whose surface is covered withthe coat having resistance to oxidation is roughened so that at leastits surface facing the anode and cathode has minute recessions andprotrusions. Then, the carbon fibers of a carbon paper constituting agas diffusion layer of the anode or cathode break the coat on the topsurface of the protruding portions of the roughened section by theclamping pressure of a cell stack, and the portions lacking the coat areelectrically connected to the anode or cathode.

In the second case, conductive particles are interposed between themetal plate whose surface is covered with a coat having resistance tooxidation and the anode or cathode in contact with this metal plate.This conductive particles break the coat on the metal plate surface bythe clamping pressure of a cell stack, and the portions lacking the coatare electrically connected to the anode or cathode. The above-mentionedconductive particles may be made contact with the metal base through thecoat by mechanically burying the conductive particles in the metal platesurface in advance.

The above-mentioned coat having poor conductivity or insulatingproperties has appropriate resistance to corrosion and a thicknesspreferably in the range of 500 Å to 10 μm for electrical connection tothe electrode.

Next, referring to FIG. 1 through FIG. 5, examples of the structure of afuel cell of the present invention will be explained. The structuralviews used here are intended to facilitate understanding, and therelative sizes and positional relations of the respective elements arenot necessarily exact.

FIG. 1 is a cross sectional view depicting essential parts of a fuelcell stack, FIG. 2 is a plan view of its anode-side separator plate, andFIG. 3 is a plan view of its cathode-side separator plate.

10 represents an electrolyte membrane and electrode assembly(hereinafter referred to as the “MEA”) composed of a solid electrolytemembrane 11, anode 12 and cathode 13 bonded to both surfaces of thesolid electrolyte membrane 11, gaskets 14 and 15 arranged on theperipheral portions thereof, etc. An anode-side separator plate 21 and acathode-side separator plate 31 are disposed on the outside of the MEA.The MEA 10 and the separator plates 21 and 31 constitute a unit cell,and a plurality of such unit cells are stacked so as to be connected inseries. In this example, a conductive metal mesh 16 and a gasket 17 areinserted between the separator plates 21 and 31 of every two cells so asto form a cooling section for passing cooling water.

The anode-side separator plate 21 is constructed by sticking a metalplate 22 shown in FIG. 4 and an insulating sheet 27 shown in FIG. 5together. The metal plate 22 is provided with an array of protrudingribs 23 formed at the center of one major surface facing the anode bypress working, and fluid inlet openings 24 a, 25 a, 26 a and fluidoutlet openings 24 b, 25 b, 26 b on the right and left. Meanwhile, theinsulating sheet 27 is fabricated by stamping a sheet, and it forms agroove 28 for guiding a fluid, i.e., a fuel gas from the fluid inletopening 24 a to the fluid outlet opening 24 b when stuck to a surface ofthe metal plate 22 having the ribs 23 and functions as a gasket forpreventing the fuel gas from leaking out of the groove 28 and forpreventing the fluid passing through the openings 25 a, 25 b andopenings 26 a, 26 b from leaking out when attached to the anode.

The groove 28 formed on the surface of the separator plate 21 causes twogrooves 23′ formed on both sides of the rib 23 by the assemblage of theribs 23 of the metal plate 22 and rib pieces 29 of the sheet 27 to passa fuel gas.

As shown in FIG. 3, the cathode-side separator plate 31 is composed of ametal plate 32 provided with an array of protruding ribs 33 formed atthe center of one major surface facing the cathode by press working andfluid inlet openings 34 a, 35 a, 36 a and fluid outlet openings 34 b, 35b, 36 b on the right and left; and an insulating sheet 37 stuck to thesurface of the metal plate having the ribs 33. Formed on the surface ofthis cathode-side separator plate 31 facing the cathode is a groove 38for guiding a fluid, i.e., an oxidant gas from the fluid inlet opening36 a to the fluid outlet opening 36 b. Besides, the sheet 37 functionsas a gasket for preventing the oxidant gas from leaking out of thegroove 38 and for preventing the fluid passing through the openings 34a, 35 a and openings 34 b, 35 b from leaking out.

The groove 38 causes four grooves 33′ formed between the ribs 33 by theassemblage of the ribs 33 of the metal plate 32 and rib pieces 39 of thesheet 37 to pass the oxidant gas.

Thus, when a separator plate is constructed by a combination of themetal plate having a plurality of ribs formed by press working and theinsulating sheet obtained by stamping, it is possible to change the sizeof the fluid passage groove by only changing the shape of the insulatingsheet.

In the above-mentioned example, the cross-sectional area of a passageformed between the ribs 33 as a gas flow path running to the groove 38of the cathode-side separator plate 31 is three times thecross-sectional area of a passage formed between the ribs 23 as a gasflow path running to the groove 28 of the anode-side separator plate 21.It is therefore possible to make the flow rate of the oxidant gasgreater than that of the fuel gas.

In the above-mentioned example, while each of the anode-side conductiveseparator plate and cathode-side conductive separator plate isindependently fabricated, it is possible to construct the anode-sideconductive separator plate and cathode-side conductive separator plateas one piece of separator plate in which one of the surfaces is ananode-side conductive separator plate and the other surface is acathode-side conductive separator plate.

EXAMPLE 1

An electrode catalyst carrying 25% by weight of platinum particles withan average particle diameter of about 30 angstroms on acetylene blackwas prepared. A dispersion of this catalyst powder in isopropanol wasmixed with a dispersion of perfluorocarbon sulfonic acid powder in ethylalcohol to form a paste. This paste was printed on one of the surfacesof a 250 μm thick carbon nonwoven fabric by screen printing so as toform an electrode catalyst layer. The amounts of platinum andperfluorocarbon sulfonic acid contained in the resultant catalyst layerwere adjusted to become 0.5 mg/cm² and 1.2 mg/cm², respectively. Byforming the catalyst layer on the carbon nonwoven fabric as a diffusionlayer in this manner, an anode and a cathode having identicalconfigurations were fabricated.

An electrolyte membrane and electrode assembly (MEA) was fabricated bybonding these electrodes to both surfaces of the center part of a protonconductive polymer electrolyte membrane having an area slightly largerthan the electrode by hot pressing so that the catalyst layers were incontact with the electrolyte membrane. The electrolyte membrane usedhere was a 25 μm thick thin film of a perfluorocarbon sulfonic acidrepresented by the following formula, wherein x=1, y=2, m=5 to 13.5, andn≈1000. Incidentally, the perfluorocarbon sulfonic acid mixed into thecatalyst layer was the same compound as the above-mentioned electrolytemembrane.

Next, a process of fabricating a conductive separator plate will bedescribed. As shown in FIG. 3, the ribs 23 with a width of about 2.8 mmand a height of about 1 mm were formed at a pitch of 5.6 mm in a 10 cm×9cm area at the center part of a 0.3 mm thick stainless steel SUS316plate by press working. Subsequently, in order to remove an oxide filmon the surface, pre-treatment was applied by sputtering with Ar under anatmosphere of Ar gas at 5×10⁻⁶ Torr. Next, under the atmosphere of Argas at 5×10⁻⁶ Torr, the temperature of this separator plate was raisedto 300° C., and an In-doped tin oxide layer was formed in a thickness of0.5 μm on the surface of the separator plate by electron beamevaporation. The metal plate 22 thus treated and the insulating sheet 27shown in FIG. 4 were stuck together so as to fabricate the anode-sideseparator plate 21. The metal plate 32 treated in the same manner andthe insulating sheet 37 were stuck together so as to fabricate thecathode-side separator plate 31.

These separator plates were combined with the above-mentioned MEAs so asto stack 50 cells, and the resultant cell stack was clamped via currentcollector plates and insulating plates by stainless steel end plates andclamping rods with a pressure of 20 kgf/cm². When the clamping pressureis too small, the gas leaks and the contact resistance between theconductive members increases, resulting in a lowering of the cellperformance. On the other hand, when the clamping pressure is too large,the electrodes are broken and the separator plates are deformed, andthus it is important to change the clamping pressure according to thedesign of the groove for the gas flow path.

As a comparative example, a cell was assembled to have the samestructure as that of Example 1 except that the conductive separatorplates were formed by stainless steel SUS316 plates with untreatedsurfaces.

Polymer electrolyte fuel cells of Example 1 and the comparative examplewere held at 85° C., and a hydrogen gas which was humidified and heatedto a dew point of 83° C. was supplied to the anode, while the air whichwas humidified and heated to a dew point of 78° C. was supplied to thecathode. As a result, an open circuit voltage of 50 V was exhibited in ano-load condition in which a current is not output.

These cells were subjected to a continuous power generation test underthe conditions of a fuel utilization ratio of 80%, an oxygen utilizationratio of 40% and a current density of 0.5 A/cm², and the changes of theoutput characteristics with time are shown in FIG. 6. As a result, theoutput of the cell of the comparative example was lowered with thepassage of time, while the cell of Example 1 retained the cell output ofabout 1000 W (22 V−45 A) over 8000 hours.

EXAMPLE 2

In this example, a cell was assembled and evaluated in the same manneras in Example 1, except for the use of separator plates having a leadoxide layer on the surface of a stainless steel plate on which Pb wasdeposited by vapor deposition.

A process of manufacturing a conductive separator plate will bedescribed below. First, after applying the same pre-treatment as inExample 1 to a 0.3 mm thick stainless steel SUS316 plate, a 1 μm thickPb layer was formed on the surface of the plate under an atmosphere ofAr (99.9999%) at 1×10⁻⁷ Torr and at a temperature of 200° C. by vapordeposition. Subsequently, a 1 μm thick PbO layer was formed on thePb-deposited surface by sputtering. The sputtering was performed underan atmosphere of Ar (99.9999%) having an oxygen partial pressure of2×10⁻⁴ Torr and at a temperature of 200° C. by controlling thesputtering power so that the film deposition rate was 3 μm/hour. Theresultant sputtered layer was identified as PbO by X-ray diffraction.The specific resistance of this PbO layer was 5×10⁻⁵ Ω·cm.

Moreover, a 1 μm thick PbO₂ layer was formed in place of PbO under anatmosphere of oxygen at 3×10⁻⁴ Torr, a temperature of 40° C. and a filmdeposition rate of 2 μm/hour.

EXAMPLE 3

In this example, a case where a nitride was used as the conductiveinorganic compound is described.

A process of manufacturing a conductive separator plate will bedescribed below. After pressing a 0.3 mm thick Ti plate in the samemanner as in the previous Example, a 1 μm thick TiN layer was formed onthe surface of the plate by sputtering using RF-planar magnetron. Thetarget used here was TiN (99%), and the sputtering was performed underan atmosphere of Ar (99.9999%) at 4×10⁻² Torr and at a temperature of500° C. by controlling the sputtering power to 400 W so that thedeposition rate was 1.5 μm/hour. The resultant sputtered layer wasidentified as TiN by X-ray diffraction. The specific resistance of thisTiN layer was 2×10⁻⁴ Ω·cm.

Incidentally, when the film thickness of TiN is reduced, there is amerit that the impedance as a cell was lowered and the outputcharacteristic is improved accordingly, but there is also a demerit thatthe long-term stability is impaired. Besides, when the film thickness ismade too thick, the reliability is increased, but it takes a longer timeto deposit a film, causing a problem that the productivity is lowered.It was found by examining the thickness of the TiN layer that athickness of about 1 μm was practical.

Next, after pressing a 0.3 mm thick Al plate in the same manner asabove, a 1.2 μm thick TiAlN layer was formed on the surface of the plateby sputtering using RF-diode. The target used here was TiAlN (99%), andthe sputtering was performed under an atmosphere of Ar (99.9999%) at4×10⁻² Torr and at a temperature of 300° C. by controlling thesputtering power to 300 W so that the deposition rate was 1.0 μm/hour.The specific resistance of the TiAlN layer fabricated by this method was1×10⁻³ Ω·cm.

EXAMPLE 4

In this example, a case where an n-type SiC, that is, a conductivecarbide was used as the conductive inorganic compound is described.

A separator plate was fabricated by forming a P-doped n-type SiC layeron a stainless steel SUS316 plate to which the same press working andpre-treatment as in Example 1 were applied. Specifically, the n-type SiClayer was formed in a thickness of 1000 angstroms on the surface of thestainless steel plate by 14.56 MHz high-frequency glow dischargedecomposition, under an atmosphere of mixed silane, methane (CH₄) anddiborane (PH₃) diluted with hydrogen in a ratio of P/(Si+C)=10 atom % at10 Torr and at a temperature of 300° C. After the deposition, a goldelectrode was vapor-deposited on the SiC layer, and the specificresistance of the SiC layer was measured 50 Ω·cm.

Cells using the separator plates of Examples 2 to 4 above were subjectedto the continuous power generation test under the same conditions as inExample 1, and the initial cell output (10 hours after the start ofoperation) and the cell output after 8000 hours operation were compared.The results are shown in Table 1.

TABLE 1 Output (W) Conductive About 8000 Cell coat Initial hours laterExample 1 Sn(In)O₂ 1200 1000 Example 2 PbO 1150  950 PbO₂ 1100  980Example 3 TiN 1220 1100 Ti—Al—N 1180 1050 Example 4 (P)SiC 1050  980

EXAMPLE 5

A nickel-plated coat containing pitch fluoride particles as a waterrepellent material was formed on the surface of a stainless steel plateSUS316L to which the same press working and pre-treatment as in Example1 were applied, under the following conditions.

Plating Bath Nickel sulphamate 150 g/l Nickel chloride 50 g/l Boric acid50 g/l Cationic surface active agent 15 g/l Tertiary-perfluoroammonium{C₈F₁₇SO₂NH(CH₂)₃N⁺(CH₃)₃ · Cl⁻} Fine particles of pitch fluoride 20 g/lpH 4.2 Bath temperature 45 ± 5° C. Anode Nickel plate Current density0.5 A/cm²

The pitch fluoride used here was prepared by finely grinding pitchfluoride with an average molecular weight of 2000 and a F/C atomicratio=1.3 into particles with an average particle diameter of 1.3 μm.The resultant nickel-plated coat had a thickness of 7 μm and Ni:pitchfluoride (weight ratio)=85:15.

This cell was subjected to the continuous power generation test underthe same conditions as in Example 1. As a result, the outputcharacteristic was almost the same as the cell of Example 1, and thecell output of 1000 W (22 V−45 A) was retained over 8000 hours.

As the thickness of the nickel-plated coat containing the pitch fluorideparticles is increased, the initial output is lowered, but the long-termreliability is improved. Moreover, as the ratio of the pitch fluoride isincreased, the initial characteristic is lowered, but the long-termreliability is improved.

Next, a separator plate was fabricated by changing the pH of theabove-mentioned plating bath and the current density duringelectrolysis, and a cell was assembled in the same manner. Incidentally,the pH of the plating bath was controlled by the amount of boric acid.

Table 2 shows the initial output and the output after about 8000 hoursoperation of the respective cells.

TABLE 2 Current Density Cell Output (W) PH (A/cm²) Initial After 8000hours 4.2  0.05 1150 1100  4.2 0.1 1180 1050  4.2 0.5 1200 1000  4.2 1.01220 950 4.2 1.1 1250 700 4.2 2.0 1260 500 1   0.5 1200 100 2.5 0.5 1200500 3   0.5 1200 900 6.5  0.05 1200 1100 

It is appreciated from Table 2 that the higher the current densityduring electrolysis, the more deterioration is caused by the long-timeuse. However, when the current density is small, an enormous processingtime is required, and the practicability is impaired. Besides, when thepH of the plating bath is smaller than 3, the deterioration is increasedby the long-time use. However, if the pH is made too close to neutral,the current density during electrolysis cannot be increased and anenormous processing time is required, impairing the practicability.

According to these results, it is appropriate to adjust the pH of theplating bath within the range of 3 to 6 and the current density per areaof a surface to be plated during electrolysis within the range of 0.1A/cm² to 1 A/cm².

EXAMPLE 6

A conductive separator plate was fabricated by forming a 7 μm thicknickel-plated layer containing fluorinated graphite particles with theuse of fluorinated graphite particles having an average particlediameter of 3 μm instead of pitch fluoride. A cell similar to that ofExample 1 was constructed by using this separator plate, and subjectedto the continuous power generation test under the same conditions as inExample 1. As a result, this cell retained the cell output of 1089 W(24.2 V−45 A) over 8000 hours.

EXAMPLE 7

In this example, plated coats were respectively formed on the stainlesssteel plates SUS316L by changing the plating metal and water repellentmaterial, and separator plates were fabricated. The outputcharacteristics of cells using these separator plates are shown in Table3.

TABLE 3 Water repellent Cell Output (W) Metal material Initial After8000 hours Nickel PTFE 1150  980 Nickel TFE-HFP 1130  960 NickelTFE-PFEV 1120  940 Gold Pitch fluoride 1200 1100 Silver Pitch fluoride1200 1020 Chrome Pitch fluoride 1200 1050 Chrome Fluorinated 1250 1060graphite

In Table 3, PTFE represents polytetrafluoroethylene (the polymerizationdegree is about 1100), TFE-HFP representstetrafluoroethylene-hexafluoropropylene copolymer (the polymerizationdegree is about 1000 and the copolymerization ratio is 1:1), andTFE-PFEV represents tetrafluoroethylene-perfluoroethyl vinyl ethercopolymer (the polymerization degree is about 1000 and thecopolymerization ratio is 1:1). The average particle diameter of theparticles of these water repellent materials is 1 μm. The pitch fluorideand fluorinated graphite are the same as those used in the above.

The plating baths used here were prepared by adding thetertiary-perfluoroammonium {C₈F₁₇SO₂NH(CH₂)₃N⁺(CH₃)₃.Cl⁻} as a cationicsurface active agent and the particles of the respective water repellentmaterials in a ratio of 15 g/l and 50 g/l, respectively, to thecompositions shown below, and electrolytic plating was performed withthe pH of 5, the bath temperature of 55±5° C. and the electrolyticcurrent of 0.2 A/cm² until the film thickness became 1 μm. The ratio byweight of the metal and water-repellent material in the resultant platedcoating was 95:5.

Gold Plating Bath Potassium dicyanoaurate 10 g/l Potassium cyanide 30g/l Potassium carbonate 30 g/l Silver Plating Bath Silver cyanide 5 g/lPotassium cyanide 20 g/l Chrome plating Bath Chromic acid anhydride 250g/l Sulfuric acid 2.5 g/l

It is apparent from these results that similar characteristics areobtainable by using any of gold, silver and chrome as well as nickel asa metal that forms the plated coat and by using any ofpolytetrafluoroethylene, tetrafluoroethylene-hexafluoropropyrenecopolymer and tetrafluoroethylene-perfluoroethyl vinyl ether copolymeras well as pitch fluoride and fluorinated graphite as a water repellentmaterial. Moreover, with the use of atetrafluoroethylene-perfluoromethyl vinyl ether copolymer (thepolymerization degree is about 1000 and the copolymerization ratio is1:1) or tetrafluoroethylene-perfluoropropyl vinyl ether copolymer (thepolymerization degree is about 1000 and the copolymerization ratio is1:1) having a similar structure to thetetrafluoroethylene-perfluoroethyl vinyl ether copolymer, similarcharacteristics were obtained.

Further, it was confirmed that, as the thickness of the plated coat andthe ratio of the water repellent material were respectively increased,the initial output was lowered, but the long-term reliability wasimproved.

EXAMPLE 8

In this example, a case where a separator plate using a spongy metal asa metal substrate is explained.

First, a spongy nickel substrate in strip form, with a METSUKE of about600 g/m² and a number of three-dimensionally communicating pores havinga substantially spindle shape with the ratio of the longer diameter tothe shorter diameter of a grid line segment substantially parallel tothe sheet surface of about 1.3, was prepared. Meanwhile, carbon blackand polytetrafluoroethylene were mixed in the ratio of 3:7 by weight,and this mixture was suspended in an aqueous solution of carboxymethylcellulose which is ten times greater in quantity so as to prepare aconductive paste. This paste was filled into the spongy nickel substrateand dried. Subsequently, the spongy nickel substrate was rolled to athickness of 0.3 mm by rollers. A section undulated at a pitch of 5.6 mm(the groove width of about 2.8 mm) was formed in a 10 cm×9 cm area atthe center part of this rolled spongy nickel substrate by press working.At this time, the height of the ribs was about 1 mm. The spongy nickelsubstrate was provided with manifold apertures for respectivelysupplying and discharging the fuel gas, cooling water and air, and wascombined with an insulating plate similar to that of Example 1 so as tofabricate the anode-side and cathode-side separator plates.

A fuel cell similar to that of Example 1 was constructed by using theabove-mentioned separator plates and subjected to the continuous powergeneration test under the same conditions as in Example 1, andconsequently the cell retained the cell output of about 1000 W (22 V−45A) over about 8000 hours.

EXAMPLE 9

With the use of separator plates fabricated from metal plates whosesurface was embossed with a variety of fine patterns by roller pressing,the effect of the roughening treatment was examined. While an aluminumseparator plate was easily embossed, it was difficult to form largerecessions and protrusions for a stainless steel separator plate, andtherefore recessions and protrusions were formed by cutting if theheight of the protruding portions was more than 100 μm. Meanwhile, theprotruding portions with a height of less than 10 μm were formed bypolishing with sandpaper, or by adjusting the coarseness of sandparticles in sand blasting. The roughened surface conditions wereobserved and confirmed with an optical stereoscopic microscope, electronmicroscope or tracer method.

Prior to testing cells after incorporating the separator plates into thecells, the contact electrical resistance between the carbon paper usedas the gas diffusion layer of the electrode and metal test piece wasmeasured with a pressure for pressing the test piece and carbon paperagainst each other as a parameter. Table 4 shows the average value ofthe contact resistance when the test piece made from stainless steelSUS316 and the carbon paper were made in contact with each other byapplication of a pressure of 25 kgf/cm².

TABLE 4 Height of Protruding Contact Resistance Metal Portions (μm) (m Ω· cm²) SUS316 Without roughening 180  treatment SUS316  5 20 SUS316  1025 SUS316  20 30 SUS316  50 55 SUS316 100 65 SUS316 200 70 SUS316 300 75SUS316 500 80

It is appreciated from Table 4 that the SUS316 plate has a large contactresistance of 120 mΩ·cm² when its surface is not roughened, but has animproved contact resistance of 10 to 50 mΩ·cm² when its surface isroughened. Regarding aluminum, after the aluminum was roughened, analumite treatment was performed by anodic oxidation. The aluminum platehas a high contact resistance of 530 mΩ·cm² if it is not roughened, buthas a contact resistance of 50 to 300 mΩ·cm² if it is roughened. In thecase of the aluminum plate, excessive anodic oxidation causes anincrease in the thickness of the alumite coating layer, and thus asignificant improvement in the contact resistance was not exhibited evenwhen the aluminum plate was roughened. It is appreciated from the aboveresults that the contact resistance tends to be smaller with a decreaseof the height of the protruding portions and that the contact resistanceis remarkably improved by roughening.

Next, in order to pursue the relationship between the configuration ofrecessions and protrusions formed on the surface of the separator plateand the contact resistance, the contact resistance was measured with theuse of separator plates on which recessions and protrusions were formedby changing the press die and press pressure in emboss press molding. Asa result, the contact resistance was not improved much when the presspressure was small and flat portions remained largely on the top of theprotruding portions. Moreover, in general, the protruding portion has awedge-like shape with a thin point, and it was found that the contactresistance is smaller as the angle of the wedge-like shape becomessmaller. Thus, it was found that the pressure for pressing the plateagainst the carbon paper or the like can be reduced for the same contactresistance. This effect was remarkable when the angle of the wedge shapewas smaller than 90 degrees.

Further, since it was found that there was a large difference in thebehavior of the contact resistance between metal plates which havesubstantially the same protruding portions on the roughened surface whenobserved by the tracer method, the causes were pursued. The finestructure of the contact section between the separator plate andelectrode was depicted in FIG. 7 based on the observation with amicroscope. Carbon fibers 1 constituting the gas diffusion layer of theelectrode were pressed against a stainless steel separator plate 2 and aconductive path is formed through an oxide coat 3 on the surface. Whenthe width W of a protruding portion 4 is narrow, it achieves a greaterimprovement in the contact resistance in comparison with a protrudingportion 4 with a wider width W. When the width of the protruding portion4 was narrower than 20 μm, the contact resistance was remarkablydecreased. As shown in FIG. 7, when the width of the protruding portion4 is substantially equal to the diameter (5 to 20 μm) of the carbonfiber 1 of the carbon paper, the deformation of the protruding portioncaused by the pressure for pressing the carbon fiber 1 and separatorplate 2 against each other increases, and the area of a substantialcontact area 6 between the carbon material and metal material becomeslarger.

Ribs for forming a gas flow path were formed by pressing a metal plateroughened as mentioned above and combined with an insulating sheetsimilar to that of Example 1 so as to fabricate separator plates. Theseseparator plates and the MEA similar to that of Example 1 were combinedto assemble a unit cell, and the power generation test was performed.The cell test was carried out as follows. First, the air and hydrogenwere humidified by putting the air into a hot water bubbler at 60 to 70°C. and hydrogen into a hot water bubbler at 80° C., and then supplied tothe manifold running to the electrodes. The test was carried out byarranging the cell temperature to be 75° C. and the gas utilizationratio indicating the ratio of the gas consumed by the electrode reactionto be 70% for hydrogen and 20% for the air. The cell performance wasevaluated by the output voltage when the load current density was 0.5A/cm².

FIG. 8 shows the performance of Cell “a” using a stainless steelseparator plate having the protruding portions whose average height andwidth were respectively about 20 μm, Cell “b” using a stainless steelseparator plate which was not roughened, and Cell “c” using a separatorplate in which grooves for the gas flow path were formed by cutting aconventional carbon plate. It was found from the results that, if thestainless steel separator plate is roughened, it can exhibit performanceas good as that of the separator plate formed by cutting the carbonplate. Moreover, the cell test was carried out for various roughenedmetal separator plates as shown in Table 4, and consequently acorrelation that the smaller the contact resistance, the higher the cellperformance was obtained. The cell performance of a cell using aluminumseparator plates was low in comparison with a cell using stainless steelbecause of the larger contact resistance, but it was found that the cellperformance can be improved by roughening the surface.

Next, the cell performance was evaluated while increasing the height ofthe protruding portions given by the roughening treatment. Stainlesssteel separator plates having the protruding portions with a height of100 μm, 200 μm, 300 μm and 500 μm, respectively, were experimentallymanufactured by press molding or cutting. While cells using any of theseparator plates exhibited good performance, some of cells usingseparator plates having the protruding portions whose height was notless than 300 μm suffered destruction of carbon paper and showed asudden lowering of the performance during the test. It is thereforepossible to say that an appropriate height of the protruding portions isnot more than 50% of the thickness of the gas diffusion layer of theelectrode.

Furthermore, the correlation between the corrosion resistance andcontact resistance of a roughened metal separator plate was examined.There were prepared some separator plates made from alloys containingiron such as stainless steel as a main component and having corrosionresistance by a passive state coat made from chrome oxide on thesurface. By mainly changing the percentage content of chrome, a pH rangein which the passive state coating was present in a stable manner wasvaried. It was found as a result of the cell tests that the cellperformance of cells using separator plates made from alloys capable ofmaintaining a high corrosion resistance even in an atmosphere where thepH was lower than 2 was not very high even when the separator plateswere roughened. On the other hand, with the use of separator plateshaving an alloy composition in which the passive state coat can not bepresent in a stable manner if the pH in the atmosphere is not higherthan 2, the cell performance was significantly improved by applying theroughening treatment. It is considered that such a significantimprovement was achieved because the effect of the roughening treatmentis enhanced as the corrosion-resistant coat on the surface is thinner toa certain extent.

EXAMPLE 10

This example will explain a case where conductive particles whosehardness is higher than that of a metal forming a metal separator plateare interposed between the gas diffusion layer of the electrode and theseparator plate.

Like Example 9, the contact resistance between the carbon paper and themetal test piece was evaluated prior to the cell test. Aluminum powder,stainless steel (SUS316) powder and cobalt powder were selected as metalconductive particles, crystalline graphite and glassy carbon wereselected as carbon conductive particles, and titanium nitride, siliconcarbide and lead oxide were selected as ceramic type conductiveparticles. After finely grinding each of these powders in an agatemortar for one hour, the resultant powder was made into a slurry with anorganic solvent, applied to the surface of the test piece, and dried tosolidify.

With the use of the carbon paper and stainless steel (SUS316) testpiece, the contact resistance was measured in a state in which they werebeing pressed against each other by a pressure of 25 kgf/cm². Theaverage value of the measured resistance is shown in Table 5.

TABLE 5 Contact Resistance Powder Material (m Ω · cm²) Without powder130-180 Aluminum 60 SUS316 40 Cobalt 30 Crystalline graphite 80 Glassycarbon 15 Titanium nitride 20 SiC 70 Lead oxide 200 

Every powder except the aluminum powder and lead oxide powder showed animprovement in the contact resistance. In particular, the stainlesssteel powder, cobalt powder, glassy carbon and titanium nitrideexhibited a significantly improved contact resistance of 15 to 40 mΩ·cm²as compared with the contact resistance of 130 mΩ·cm² when such a powderwas not used.

In order to determine the cause of the improvement in the contactresistance, after the test, the surface of the stainless steel testpiece was observed with a microscope. As a result, in the case where theglassy carbon or titanium nitride powder was used, innumerable scratcheson the stainless steel surface were observed. On the other hand, in thecase where the aluminum powder or lead oxide powder was used, suchscratches were not found. It is considered from these results thatconductive fine particles are present on the contact surface in such astate that the particles break the coat on the surface of the stainlesssteel and ensure electrical conductivity between the carbon fibersconstituting the carbon paper and the stainless steel test piece.

In the case where a crystalline graphite powder which was a carbonconductive particle but had a hardness smaller than the Vickers hardness(180 to 220 HV) of stainless steel was used, the improvement in thecontact resistance was small. Moreover, since carbides have high Vickershardness, when a carbide having higher conductivity than a siliconcarbide used in the experiment was used, the contact resistance wassignificantly improved. Furthermore, with the use of the aluminum andlead oxide powders, the contact resistance was not improved because thehardness of the aluminum and lead oxide powders was lower than that ofstainless steel and the conductivity of the oxide coat on the surfaceand their own conductivity were low.

Next, the cell tests were carried out using the glassy carbon powder andstainless steel powder among these conductive powders with highhardness. The basic structure of the cell, such as the electrolytemembrane and electrodes, the manufacturing and assembling procedure weresubstantially the same as those of Example 9. The glassy carbon powderwhose Vickers hardness was 550 HV was obtained by grinding a block ofglassy carbon powder prepared by heating a thermosetting resin over along time, with a ball mill. The average particle diameter of thiscarbon powder was 30 μm. Besides, SUS304 powder having the averageparticle diameter of 30 μm was used as the stainless steel powder.During the assembly of a fuel cell, the glassy carbon powder which wasmade into a slurry with ethanol was applied to the stainless steel(SUS316) separator plate's surface in contact with the electrode, anddried. The pressure for pressing the separator plate and electrodeagainst each other was adjusted to 25 kgf/cm².

The cell test was carried out under the same conditions as in Example 9.FIG. 9 shows the performance of Cell “d” using a separator plate onwhich the glassy carbon powder was applied, Cell “e” using a separatorplate on which the stainless steel powder was applied, and Cell “c” ofthe comparative example used in Example 9. It will be appreciated thatCells “d” and “e” yielded a significantly improved output voltage of0.63 to 0.65 V at the current density of 0.5 A/cm² as compared with thevalue of 0.50 V of the cell of the comparative example. Moreover, with acell using a separator plate on which crystalline carbon was applied, aslight improvement of the output voltage to 0.57 V was confirmed.

In a cell in which the glassy carbon powder is coated on the stainlesssteel separator plate, glassy carbon particles 8, which are fixed to anorganic binder 7, pierce an oxide coat layer 3 and reach a metal base 2while making contact with the carbon fibers 1 of the gas diffusion layerof the electrode, at the contact surface with the electrode, asschematically shown in FIG. 10. Since the conductive paths are formed bythis glassy carbon particles 8, the contact resistance is significantlyimproved.

As described above, it is appreciated that the cell performance isimproved as the value of the contact resistance becomes smaller. In thisexample, while the SUS304 powder which is as hard as or slightly softerthan SUS316, that is the material of the separator plate, was used asthe stainless steel powder, the cell performance is further improved byusing a harder stainless steel material or metal powder.

Next, powders having a variety of average particle diameters (5 μm, 10μm, 20 μm, 35 μm and 60 μm) were prepared by changing the conditions ofgrinding the glassy carbon to be applied to the separator plate, and therelationship between the particle diameter and the cell performance wasexamined. All of the cells showed an improvement in the performance atthe early stage of operation, but the performance of a cell using apowder with a particle diameter greater than 20 μm tended to lowergradually during the cell test over 200 hours. It is considered thatsuch a lowering of the performance is caused because the carbonparticles are hard to be retained on the contact surface when thediameter of the carbon particles is larger than the diameter (5 to 20μm) of the carbon fiber as a porous carbon material of the electrode.

In the above example, the carbon particles were made into a slurry withthe organic solvent and then applied to the surface of the metalseparator plate, but the adhesive strength of the carbon particles wasweek and sometimes the carbon particles dropped during the assembly of acell. Therefore, the glassy carbon powder was dispersed in an ethanolsolution of 2% by weight of polyvinyl butyral to form a slurry, and theslurry was coated on the separator plate and dried. Moreover, in orderto increase the conductivity of the coated film, a slurry was preparedby adding 5 to 50% by weight of crystalline carbon powder to the glassycarbon powder and adding a surface active agent to impartdispersiveness, and then applied to the separator plate and dried. Acell using the separator plate thus obtained was tested. As a result,when polyvinyl butyral was merely added to the slurry of the glassycarbon powder, the performance was slightly lowered. However, when theconductivity of the coated film was increased by further adding thecrystalline carbon, the performance was improved. It was possible toprevent the carbon powder from being dropped during the assembly byadding a binder. Moreover, it was found that, when a crystalline carbonpowder or the like is added to improve the conductivity of the coatedfilm, if around 5% by weight of conductive particles having highhardness is contained, it produces the effect of improving the contactresistance.

Furthermore, it was found, as a result of performing the cell test usinga stainless steel separator plate roughened by sand blasting so as tostrengthen the adhesiveness of the coated film and increase the contactarea, that the cell performance was apparently improved in comparisonwith a cell using a separator plate which was not sand-blasted. Thereason for this is considered that polyvinyl butyral shrunk during theevaporation of ethanol and solidification and the force pressing theglassy carbon particles against the surface of the stainless steel wasincreased.

As another method of forming conductive particles with high hardness onthe contact surface, the following method was examined. Specifically,after dispersing the powder of glassy carbon on the metal surface, itwas mechanically pushed into the metal plate by roller pressing. It wasconfirmed by observing the surface with a microscope that a number ofcarbon particles are buried in the metal surface. A press pressure of 50kg-force/cm² was sufficient for soft metals such as aluminum, but apress pressure of not less than 100 kg-force/cm² was required forstainless steel. Further, it was confirmed by an experiment in whichrelatively coerce glassy carbon particles of around 200 μm were causedto strike the metal surface by the pressure air that broken pieces ofthe glassy carbon partly remained on the metal surface.

Besides, around 10 wt % of the glassy carbon ground to a particlediameter of around 10 to 100 μm was mixed into melted aluminum anddispersed. The mixture was cooled down while applying ultrasonicvibration so as to prevent segregation when solidified, and a compositeblock of the glassy carbon powder and aluminum was obtained. A metaltest piece to be used for measuring the contact electrical resistancewas cut out, and soaked in hydrochloric acid for 1 to 5 minutes.Thereafter, the alumite treatment was applied to the surface by anodicoxidation to impart a corrosion resistance, and then the contactresistance was measured. The contact resistance was sufficiently smallranging from 10 to 30 mΩ·cm². It was found by observing the surface witha microscope that a number of glassy carbon particles pieced an aluminafilm formed by the anodic oxidation and appeared on the surface. It waspossible to obtain metals with small contact resistance and highcorrosion resistance by using metals other than aluminum, for example,stainless steel, by the same method.

A cell test was carried out using a stainless steel separator plate withsuch glassy carbon particles mechanically buried in its surface incontact with the electrode, and consequently good cell characteristicswere exhibited like a cell using a separator plate on which the glassycarbon particles that were made into a slurry with an adhesive agent wascoated.

In the above example, while the fine particles of glassy carbon wereused as the conductive particles with high hardness, needless to say, itis similarly effective to use other conductive particles with highhardness. Besides, the conductive particles with high hardness areparticularly effective for metals having resistance to corrosion becauseof a film with low conductivity, such as a passive state film of metaloxide and alumina film.

In each of Examples 9 and 10, a unit cell was tested, but when a cellstack is to be tested, a cooling water section is formed for every twoto three cells so as to collect the generated Joule heat and to keep acertain temperature of the cell stack. In this case, it is necessary tocontrol the electrical resistance at the contact section between themetal separator plates. Then, the contact resistance between theseparator plates can be lowered by roughening the surface of the metalseparator plates and providing conductive particles having higherhardness than the separator plates on the surface of the metal separatorplates.

INDUSTRIAL APPLICABILITY

According to the present invention, for the separator plate, since ametal material such as stainless steel can be used without cutting,instead of a conventional method of cutting a carbon plate, asignificant reduction in the cost can be achieved for mass production.Moreover, since the separator plate can be made thinner, it contributesto the realization of a compact cell stack.

We claim:
 1. A solid polymer electrolyte fuel cell comprising: a solidpolymer electrolyte membrane; an anode and a cathode sandwiching saidsolid polymer electrolyte membrane therebetween; an anode-sideconductive separator plate having a gas flow path for supplying a fuelgas to said anode; and a cathode-side conductive separator plate havinga gas flow path for supplying an oxidant gas to said cathode, whereinsaid anode-side and cathode-side conductive separator plates are formedof a metal whose surface is covered with a coat having resistance tooxidation, at least surfaces in contact with the anode and cathode areroughened to have recessions and protrusions, and portions of a topsurface of protruding portions, which lack said coat, are electricallyconnected to the anode and cathode, respectively.
 2. A solid polymerelectrolyte fuel cell comprising: a solid polymer electrolyte membrane;an anode and a cathode sandwiching said solid polymer electrolytemembrane therebetween; an anode-side conductive separator plate having agas flow path for supplying a fuel gas to said anode; and a cathode-sideconductive separator plate having a gas flow path for supplying anoxidant gas to said cathode, wherein said anode-side and cathode-sideconductive separator plates are formed of a metal whose surface iscovered with a coat having resistance to oxidation and whose surfacesfacing the anode and the cathode have portions which lack said coat andare electrically connected to the anode and the cathode, respectively,through conductive particles interposed between said separator plate andthe anode and between said separator plate and the cathode.
 3. The solidpolymer electrolyte fuel cell as set forth in claim 2, wherein saidconductive particles are formed from a material whose hardness is higherthan that of the metal constituting said conductive separator plates. 4.The solid polymer electrolyte fuel cell as set forth in claim 3, whereina surface of each of said anode and cathode that faces said conductiveseparator plate is made of a porous layer containing-carbon particles orcarbon fibers, and a particle diameter of said conductive particles issmaller than a particle diameter of said carbon particles or a diameterof said carbon fibers.
 5. The solid polymer electrolyte fuel cell as setforth in claim 3, wherein said conductive particles are coated on asurface of each of said separator plates or a surface of each of saidanode and cathode.
 6. The solid polymer electrolyte fuel cell as setforth in claim 1, wherein said anode-side conductive separator plate iscomposed of a metal plate having grooves or ribs for guiding the fuelgas on its surface facing said anode, and an insulating sheet whichforms a gas flow path for guiding the fuel gas from a gas supply side toa gas discharge side on a surface of said metal plate in cooperationwith said grooves or ribs and which has elasticity to function as agasket for preventing the fuel gas from leaking out of said gas flowpath; and said cathode-side conductive separator plate is composed of ametal plate having grooves or ribs for guiding the oxidant gas on itssurface facing said cathode, and an insulating sheet which forms a gasflow path for guiding the oxidant gas from a gas supply side to a gasdischarge side on a surface of said metal plate in cooperation with saidgrooves or ribs and which has elasticity to function as a gasket forpreventing the oxidant gas from leaking out of said gas flow path. 7.The solid polymer electrolyte fuel cell as set forth in claim 2, whereinsaid anode-side conductive separator plate is composed of a metal platehaving proves or ribs for guiding the fuel gas on its surface facingsaid anode and an insulating sheet which forms a gas flow path forguiding the fuel gas from a gas supply side to a gas discharge side on asurface of said metal plate in cooperation with said grooves or ribs andwhich has elasticity to function as a gasket for preventing the fuel gasfrom leaking out of said gas flow path; and said cathode-side conductiveseparator plate is composed of a metal plate having grooves or ribs forguiding the oxidant gas on its surface facing said cathode and ainsulating sheet which forms a gas flow path for guiding the oxidant gasfrom a gas supply side to a gas discharge side on a surface of saidmetal plate in cooperation with said grooves or ribs and which haselasticity to function as a gasket for preventing the oxidant gas fromleaking out of said gas flow path.
 8. A solid polymer electrolyte fuelcell comprising: a solid polymer electrolyte membrane; an anode and acathode sandwiching said solid polymer electrolyte membranetherebetween; an anode-side conductive separator plate having a gas flowpath for supplying a fuel gas to said anode; and a cathode-sideconductive separator plate having a gas flow path for supplying anoxidant gas to said cathode, wherein each of said anode-side andcathode-side conductive separator plates is composed of a spongy metaland a carbon powder layer which is filled into said spongy metal andcoated on a surface of said spongy metal.
 9. The solid polymerelectrolyte fuel cell as set forth in claim 8, wherein the spongy metalcomprises a spongy nickel.
 10. A solid polymer electrolyte fuel cellcomprising: a solid polymer electrolyte membrane; an anode and a cathodesandwiching said solid polymer electrolyte membrane therebetween; ananode-side conductive separator plate having a gas flow path forsupplying a fuel gas to said anode; and a cathode-side conductiveseparator plate having a gas flow path for supplying an oxidant gas tosaid cathode, wherein each of said anode-side and cathode-sideconductive separator plates is composed of a metal and a conductive coatwhich has resistance to oxidation and covers a surface of the metal,wherein said conductive coat is selected from the group consisting of acarbonaceous coat, a metal-plated coat containing particles of a waterrepellent material, and a conductive inorganic compound coat wherein theconductive inorganic compound is selected from the group consisting ofSn(In)O₂, PbO, PbO₂, and inorganic carbides.
 11. The solid polymerelectrolyte fuel cell as set forth in claim 10, wherein said conductivecoat is a metal-plated coat containing particles of a water repellentmaterial, and the water repellent material is selected from the groupconsisting of pitch fluoride, fluorinated graphite,polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylenecopolymer, and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer.12. The solid polymer electrolyte fuel cell as set forth in claim 10,wherein said conductive coat is a metal-plated coat containing particlesof a water repellent material, and the metal of said metal-plated coatcomprises a metal selected from the group consisting of gold, silver,nickel, and chrome.
 13. The solid polymer electrolyte fuel cell as setforth in claim 10, wherein the particle diameter of the particles ofwater repellent material is in a range of 0.05 to 50 μm.
 14. The solidpolymer electrolyte fuel cell as set forth in claim 10, wherein themetal-plated coat containing particles of water repellent material has athickness in a range of 0.5 to 10 μm.
 15. The solid polymer electrolytefuel cell as set forth in claim 10, wherein said anode-side andcathode-side conductive separator plates are formed as a one pieceseparator plate, wherein a first surface of said one piece separatorplate forms the anode-side conductive separator plate and a secondsurface of said one piece separator plate forms the cathode-sideconductive separator plate.
 16. The solid polymer electrolyte fuel cellas set forth in claim 10, wherein said anode-side conductive separatorplate is composed of a metal plate having grooves or ribs for guidingthe fuel gas on its surface facing said anode and an insulating sheetwhich forms a gas flow path for guiding the fuel gas from a gas supplyside to a gas discharge side on a surface of said metal plate incooperation with said grooves or ribs and which has elasticity tofunction as a gasket for preventing the fuel gas from leaking out ofsaid gas flow path; and wherein said cathode-side conductive separatorplate is composed of a metal plate having grooves or ribs for guidingthe oxidant gas on its surface facing said cathode and an insulatingsheet which forms a gas flow path for guiding the oxidant gas from a gassupply side to a gas discharge side on a surface of said metal plate incooperation with said grooves or ribs and which has elasticity tofunction as a gasket for preventing the oxidant gas from leaking out ofsaid gas flow path.
 17. The solid polymer electrolyte fuel cell as setforth in claim 10, wherein the metal of said separator plates isselected from the group consisting of aluminum, stainless steel andnickel.